Honeycomb structures are in use, for example, as a carrier for the catalyst used in an internal combustion engine, a boiler, a chemical reactor, a reformer of fuel cell or the like, or as a filter for capturing fine particles present in an exhaust gas, particularly fine particles emitted from a diesel engine (the filter is hereinafter referred to as DPF).
In general, the honeycomb structure used for such a purpose is constructed, as shown in FIG. 1(a) and FIG. 1(b), in such a way that it has a plurality of cells 23 divided from each other by partition walls 24 and functioning as a passage for fluid, each cell 23 is plugged at either of the two ends so that adjacent cells 23 are plugged alternately at each end face of the honeycomb structure to make each end face a checkered pattern. In the honeycomb structure 21 having such a construction, a subject fluid enters into those cells 23 not plugged at the inlet side end face 25 of the structure 21, i.e. those cells 23 plugged at the outlet side end face 26 of the structure 21, passes through the partition walls 24, and is discharged from adjacent cells 23, i.e. those cells 23 plugged at the inlet side end face 25 and not plugged at the outlet side end face 26. In this case, the partition walls 24 function as a filter and, when the honeycomb structure 21 is used, for example, as a DPF, the soot, etc. discharged from a diesel engine are trapped by the partition walls 24 and deposited on the partition walls 24.
In the honeycomb structure 21 used as above, the sharp temperature change of exhaust gas and the local heating make non-uniform the temperature distribution inside the honeycomb structure 21, which has caused problems such as crack generation in honeycomb structure 21 and the like. When the honeycomb structure 21 is used particularly as a DPF, it is necessary to burn the fine carbon particles deposited on the filter to remove the particles and regenerate the filter. In that case, high temperatures are inevitably generated locally in the filter; as a result, there have easily appeared a reduction in regeneration efficiency due to the non-uniformity of regeneration temperature and crack generation due to a big thermal stress.
Hence, it was proposed to bond a plurality of honeycomb segments with a bonding agent to produce a honeycomb structure. Specifically, in, for example, U.S. Pat. No. 4,335,783 is disclosed a process for producing a honeycomb structure, which comprises bonding a large number of honeycomb parts A (see FIG. 2) using a discontinuous bonding agent B.
Also in, for example, JP-B-61-51240 is proposed a heat shock-resistant rotary regenerating heat exchanging method which comprises forming, by extrusion, matrix segments of honeycomb construction made of a ceramic material, firing them, making smooth, by processing, the outer peripheral portions of the fired segments, coating the to-be-bonded areas of the resulting segments with a ceramic bonding agent having, when fired, substantially the same mineral composition as the matrix segments and showing a difference in thermal expansion coefficient, of 0.1% or less at 800° C., and firing the coated segments.
Further in, for example, a SAE article 860008 of 1986 is disclosed a ceramic honeycomb structure obtained by bonding cordierite honeycomb segments with a cordierite cement.
In the filter obtained by bonding a plurality of honeycomb segments into one piece, it is important to make sufficient a bonding strength between honeycomb segments. It is presumed that such a bonding strength is exhibited by an anchor effect which is generated by entering of particles contained in the bonding agent used, into the surface unevenness of the outer wall of each honeycomb segment. As a relevant technique developed by focussing on such an anchor effect, there is disclosed in, for example, JP-A-2000-279729, a honeycomb filter in which the surface roughness Rz of the outer wall of honeycomb segment is specified. However, when the specification of the surface roughness (absolute value) of the outer wall of honeycomb segment restricts the physical properties (e.g. pore diameter and porosity) of honeycomb segment or makes it difficult to obtain an intended pore diameter, porosity, etc., a step of processing by spray or the like becomes necessary in order to achieve a specified surface roughness (Rz) of outer wall; thus, there has been, for example, a problem of an increase in the number of production steps.